Blood-taking devices using vacuum suction have recently replaced the piston syringes that have been conventional for such purposes. However, the suction level cannot be fine-controlled when sucking the blood into an evacuated blood-sample container. Once the needle has pierced the stopper, suction is at full vacuum. While advantageous for other purposes, this condition is a drawback if the needle did not hit the vein but instead is located in the adjoining tissue. If this happens, the full vacuum produces a hematoma in the tissue which is disagreeable to the patient. Moreover, an evacuated blood-sample container was opened and vented without pulling in blood and must be replaced by a new one in the next attempt to puncture a vein.
Accordingly blood-taking devices of the above kind comprise a blood-observation chamber outside the blood-sample container and allowing to recognize the blood flowing out of the vein before vacuum is applied to the needle, i.e., before the needle has pierced the stopper. Because of the present higher sanitary requirements entailed by AIDS, hepatitis, etc., demanding total avoidance of patient blood which might contaminate the service personnel, it is necessary to design the blood-observation means as a chamber.
If necessary, such a blood-taking device allows hitting the tissue several times until the sought vein has been pierced. This is ascertained by blood entering the blood-observation chamber. Thereupon the needle is made to pierce the stopper.
A blood-taking device is known from the state of the art, namely from the German patent 28 35 101. In this known design, the blood-observation chamber is present at the middle of the needle. For that purpose the needle is made in two parts. Contrary to the known designs of other kinds, in this design the blood is sucked into the observation chamber. For that purpose the needle is pushed through the stopper into a suction chamber subjected to vacuum by means of a displaceable membrane, said suction chamber sucking the blood with a design-limited suction effect into the blood-observation chamber.
This known design suffers from the drawback that, on one hand, the manufacturing costs are high due to the complex needle construction. However, even when the physician operates the known blood-taking device, substantial drawbacks are incurred due to the blood-observation chamber being mounted on the needle. As a rule several blood-sample containers must be filled consecutively for the different blood tests. The needle is left in the patient's vein and the blood-sample containers are applied consecutively. However, when the blood-sample container is being changed, the blood remains stationary for some time in the blood-observation chamber and begins to clot.
This may lead to clogging, as a result of which the suction of the next applied blood-sample container no longer suffices to suck in blood. Moreover, the partly clotted blood from the blood-observation chamber may render ineffective the typically present anti-coagulant in the next applied blood-sample container, leading to spuriousness in test results.
In the light of the above, the above state of the art was not accepted in practice.